Dose, and therefore cost, depends on cat weight. A bottle will last Dumps over 100 days as the stuff keeps for months. Fewer vet visits are needed because there is no risk of hypoglycaemia, so it sort of evens out.
It was only approved last year and can only be given to "happy diabetic" cats that are insulin-resistant but not insulin-deficient (i.e. Type 2 diabetes), have no ketones in their urine, and have never had insulin therapy. There are human equivalents, which are also newish.